The present invention relates to a logic circuit, such as an inverter circuit, which is formed by a junction type or a Schottky barrier gate type field effect transistor (FET).
Recently, the use of integration circuits utilizing a gallium-arsenic (GaAs) substrate instead of a silicon (Si) semiconductor substrate has become wide-spread due to their rapid operation characteristic. However, the use of the GaAs substrate gives rise to several problems, for example, the manufacture of MOS type FET's is difficult, and thus the MOS FET's must be replaced by junction type or Schottky barrier gate type FET's. MOS FET's are widely used in the conventional Si substrate at present, because this enables an enhancement type FET to be easily manufactured.
Usually, depletion type and enhancement type FET's are used as load resistors in the inverter circuits widely used in logic circuits, but the enhancement type FET is more usually used as a driver transistor. However, the junction type FET can be transformed into an enhancement type FET (strictly speaking, the normally OFF type FET) by decreasing the total amount of impurities therein.
Some problems arise in the design and manufacture of the logic circuit using the junction type FET. For example: (1) it is necessary to strictly control the threshold voltage of the FET's (.+-.0.01 V), and (2) the logic amplitude cannot be increased, because a forward voltage larger than a built-in voltage (0.7 V in the GaAs Schottky type junction) cannot be applied to a gate junction. However, the use of the junction type FET has the advantages that (1) the logic amplitude is small and thus the electric power dissipation is low and the operation speed becomes high, and (2) an ultra high speed IC can be obtained by using a material other than Si for the semiconductor substrate.
However, as shown hereinafter in detail, in the inverter circuit formed by the junction type FET, the output level depends on an electric source voltage, and when the logic levels between an input and an output become adjusted, the electric source voltage cannot be increased. Therefore, a characteristic similar to that of the MOSFET cannot be obtained.